There are relatively few effective, environmentally acceptable options for disposal of hazardous waste compositions and more particularly hazardous waste compositions containing electric arc furnace dust (EAFD). Recycling is clearly the most desirable "disposal" option for any waste, but this option is extremely limited for hazardous waste. Incineration of waste is highly desirable if the wastes are combustible and even more so if wastes are in sufficient quantities that by-product energy from the combustion can be captured. Incineration typically yields some non-combustible residue.
The ultimate disposal method for nearly all waste residues is in or on the surface of the earth by way of landfilling. Focusing specifically on hazardous wastes, these wastes must be isolated from the environment, through treatment or fixation/stabilization or both, to eliminate or minimize any future environmental impact. For hazardous wastes which are placed in landfills, a highly desirable option is fixation/stabilization, in which wastes are converted to a monolithic mass having good structural integrity, a low permeability coefficient and reduced leachate potential.
U.S. Pat. No. 5,245,122, entitled METHOD AND MIXTURE FOR TREATING ELECTRIC ARC FURNACE DUST, discloses a method for chemically stabilizing a hazardous waste composition containing EAFD by using the pozzolanic characteristics of EAFD. This method involves forming a mixture of EAFD with water and lime and, optionally, ferrous sulfate. The freshly blended product has acceptable leachate concentrations. The method disclosed in the '122 patent minimizes the concentration of certain heavy metals in the leachate from the freshly blended product.
The concentration of certain heavy metals in the leachate is of specific significance because the Environmental Protection Agency (EPA) demands that hazardous waste disposal compositions do not release heavy metals even after exposure to acid rainfall. Acidic rainfall is known to be prevalent throughout much of the United States, partly due to man-made air pollution such as sulfur and nitrogen oxides from combustion and fossil fuels, and partly due to the inclusion of atmospheric carbon dioxide in precipitation. The EPA has established specific test protocols, which the fixated/stabilized waste materials must pass if the processing is to be considered environmentally suitable. These test protocols relate to the effect of acidic rainfall on landfill disposed waste.
In particular, all three of the tests that have been developed by the EPA are predicated upon exposing the landfill compositions to acid, followed by an analysis of the fluids generated from that test with respect to the toxic metals content.
In particular, EPA SW-846 TEST METHODS FOR EVALUATING SOLID WASTE: PHYSICAL/CHEMICAL METHODS, Third Edition, contains:
(1) Method 1310-EXTRACTIVE PROCEDURE TOXICITY TEST (EPTT) METHOD; PA1 (2) Method 1311-TOXICITY CHARACTERISTIC LEACHING PROCEDURE (TCLP); and PA1 (3) Method 1320-MULTIPLE EXTRACTION PROCEDURE.
The EPTT method involves exposing the material to be tested to an acetic acid solution, which has been deemed equivalent to acid rainfall exposure. The EPTT method has been superseded by the TCLP, which is a more rigorous acetic acid procedure. The multiple extraction procedure is a method of evaluating long term acid rainfall by exposure to a combination of nitric and sulfuric acids in nine sequential tests.
The effect of acidity on a fixated/stabilized mass is to lower pH levels within the mass. This tends to allow many heavy metals (e.g., lead, nickel, chromium) to be re-solubilized in water. Indeed, many of the heavy metals tend to achieve minimum solubility within a fairly tight pH range, that is they re-solubilize at pH levels below or, in some cases, above that range. This pH range, over which heavy metals experience minimum solubility, varies depending on the metal and on the source of the waste composition. Of course, if permitted to solubilize, the heavy metals tend to leach out of the fixated/stabilized mass, to the detriment of the environment.
Although typical fixated/stabilized compositions, such as compositions containing lime, Portland cement or class "C" fly ash, are alkaline in nature, acid rain or acid rain simulating tests can overcome this alkalinity. One approach to avoid the decrease in pH resulting from exposure to acid rain would be to place additional quantities of an alkaline agent, such as lime or limestone, into the waste compositions. If such an alkaline agent were added, however, the pH level in the composition would tend to be shifted up or down, toward a pH value of 8 if the agent is limestone or toward a pH value of 12 if the agent is lime. Both of these values are outside of the target pH range for minimizing the solubility of heavy metals. Therefore, the simple inclusion of alkaline agents such as lime or limestone does not aid in preventing heavy metals from solubilizing.
Thus, there is a need for a buffering agent which is capable of maintaining a hazardous waste composition within a pH range over which heavy metal solubility tends to be minimized.
Furthermore, it would be advantageous for this buffering agent to be capable of maintaining this pH range over a relatively wide range of lime content in the waste, which otherwise affects the pH of the waste composition. There are several reasons for a variation in the lime content. For example, the waste composition might not be completely well mixed which would cause areas of high and low concentration of lime. Also, with respect to EAFD wastes, the concentration of lime varies depending on the source of the waste. Also, the available calcium oxide content of EAFD from some sources tends to vary widely with time. In addition, it might be difficult to precisely proportion the various components of the composition, such as ferrous sulfate and lime. The buffering agent should preferably be available as a fine, dry material for ease of proportioning and mixing.
Finally, although buffers are commonly known as soluble materials, a buffering agent for use in the present invention should be essentially insoluble in water. Soluble buffers in fixation/stabilization systems would rapidly dilute or leach away from the mass upon rainfall, thereby rendering such buffering agents useless to maintain the desired pH range for such systems.